1. Field of the Invention
The present invention relates to a light-emitting device used in the automatic focus adjustment apparatus for a video camera or still camera.
2. Description of the Prior Art
Various systems have been proposed for the automatic focus adjustment apparatus which drives the image forming lens to be in a focusing position. Among them is a system which emits light to an object and detects reflected light from the object for focus adjustment (hereinafter referred to as an active system). U.S. Pat. No. 4,441,810 and European Patent Publication EP No. 0,140,650 Al disclose this type of system; a light emitter and a photo sensor are positioned a specified distance apart from each other based on the principle of trigonometrical survey. Light from a light-emitting device is collected by a condenser lens and projected to an object. Reflected light from the object is condensed by a receiving lens and fed to the photo sensor, which photoelectrically detects the position of the centroid of the image formed by the reflected light to obtain distance imformation for focus adjustment.
The focus adjustment apparatus of the active system described above is based on the assumption that, when the distance between the apparatus and the object is fixed, the centroid of the spot image formed on the photo sensor by the reflected light is also fixed.
In the focus adjustment apparatus of the active system, the light-emitting device is normally an infrared ray-emitting diode (hereinafter abbreviated as IRED). Light from this device is collected by a condenser lens and then projected onto the object. At the same time, light reflected by the inner walls of the outer casing and the surface of the base of the device are also projected through the condenser lens onto the object; direct light forms a high intensity infrared light image at the center, surrounded by a lower intensity infrared light image (hereinafter referred to as a flare image) formed by light reflected from the inner walls of the outer casing and the surface of the base (hereinafter called flare). If the object has constant reflectance over its entire surface, the centroid of the spot image on the photo sensor is virtually fixed irrespective of the flare due to the flare's negligibly low light intensity. If the object has different reflectances, however, problems arise. Suppose that a higher intensity image is formed by direct light on a low reflectance area of the object while a lower intensity flare image is partly formed on a high reflectance area. In such a case, the light image formed on the sensor will have intensity distribution different from that for a constant reflectance subject. Since the flare image light intensity would be significantly high, the position of the centroid of the spot image would change. In other words, though the distance from the object is constant, the position of the centroid of the spot image changes on the photo sensor, resulting in an out-of-focus picture.